Due to request for low abrasion on rotational elements to achieve an extended life and for low extent of noise, fluid dynamic bearings (FDBs) have been used in fan motors and hard disk drive motors.
As shown in FIG. 4, in an FDB, a rotary shaft 5 is rotatably inserted into a sleeve 3 with a bearing clearance formed between the rotary shaft 5 and the sleeve 3. A dynamic pressure generating groove 3A is formed on an inner peripheral surface of the sleeve 3. Lubricating oil is applied to the bearing clearance. A pressure is generated due to the pumping action of the dynamic pressure generating groove 3A caused by the rotation of the rotary shaft 5. As a result, the rotary shaft 5 rotates in the sleeve 3 without radial contact with the sleeve 3.
An example of a conventional apparatus for manufacturing this type of groove-equipped FDB is described below with reference to FIG. 5. As a conventional method for processing the dynamic pressure generating groove 3A, a method for plastically processing the dynamic pressure generating groove 3A by using hard balls is known. An apparatus for carrying out such a method is shown in FIG. 5. In this apparatus, a guide shaft 14 defines a guide hole 142 through a leading end of the guide shaft 14. Three balls 15 are held radially into the guide hole 142. The diameter of the balls is so selected that the sum of the diameter of the balls is slightly greater than an inner diameter of the sleeve 3.
In operation of the conventional apparatus, a rotational speed (W) and a feeding speed (V) with respect to the sleeve 3 are simultaneously applied to the guide shaft 14, the dynamic pressure generating groove 3A resembling the motion of the hard balls 15 is thus formed by plastic processing. If processing the dynamic pressure generating groove 3A of a herringbone type as shown in FIG. 4, when the balls 15 have been fed to the approximate center of the sleeve 3, a half of the pressure generating groove 3A slanting in one direction with respect to the axis of the sleeve 3 is formed. The rotational direction of the guide shaft 14 is then inverted without changing the feeding speed (V) of the guide shaft 14 to process the other half of the pressure generating groove 3A slanting in another direction with respect to the axis of the sleeve 3.
Since the pressure generating groove 3A is a result of the motion of the hard balls 15, a high manufacturing precision of the apparatus is required to achieve a high accuracy of the pressure generating groove 3A. For example, a high consistency of concentricity of a tooling portion of the apparatus must be satisfied, that is, a circumscribed circle about the hard balls 15 is required to be highly concentric with the guide shaft 14. In the apparatus as described above, however, the hard balls 15 are separately manufactured and then assembled to the guide shaft 14. These separate components impose difficulty to manufacture the apparatus with high precision, since it is required not only that the discrete components be precisely manufactured, but also that the discrete components be precisely assembled.
Therefore, a fluid dynamic bearing manufacturing tool which has a high precision and is relatively easier to be manufactured is desired.